The correct order of the decreasing ionic radii among the following isoelectronic species is:

For isoelectronic species (all have 18 electrons):Ionic radius is inversely proportional to the atomic number ($Z$). More protons pull the same number of electrons closer to the nucleus.${\text{S}}^{2-}(Z=16)$${\text{Cl}}^{-}(Z=17)$${\text{K}}^{+}(Z=19)$${\text{Ca}}^{2+}(Z=20)$ Order of decreasing radii: ${\text{S}}^{2-}>{\text{Cl}}^{-}>{\text{K}}^{+}>{\text{Ca}}^{2+}$.